Ending the pot discussion????

[Paul Schreiber]

OK, let chat on pots some more (need a break).

Here is a 'checklist' that I did in MOTM. This is typical of how I chose all of my parts, for MOTM or anything else.
In this case, it will be MOTM specific. The choice are in order of how I arrived at the Spectrol 149.

1) Needs to have a stainless steel shaft, not plastic. Shaft length needs to be .425 to .500

This is so the shafts won't break off, and so the knobs will be above the surface of the panel. 

2) Needs to have cast bushing (the threaded part).

This is for strength. In MOTM, each pot is bolted to the panel. Since a wide variety of users will be tightning,
I did not want plastic bushings. Too easy to strip out.

3) Right angle, pcb mount.

Here is where I start to reduce my choices. The MOTM pcbs are soldered to the pots to reduce wiring, and
wiring errors. This is a design decision. Now, some will say this is bad, because "it makes the pots hard to
replace". Well, it depends on what you are calling 'hard'.

A MOTM pot swap takes about 7 minutes longer. But, since this is a rare occurance, I did what all
design engineer's do: I 'traded off' the extra time (5 years from now) with ZERO chance for wiring errors,
and about 45 minutes LESS time in initial construction.

I think this is a valid trade-off. Nothing more frustrating that de-bugging backwards pot wires from a pc board
up to the panel. Nine combinations, only 1 correct. And this doesn't count 'daisy chaining' wires pot-to-pot,
and getting that wrong. All you ASM builders: how long did it take to wire up CORRECTLY all those panel pots?
And the issue of 'dressing' the wires all nice and neat, all ty-wrapped together. More time, and more costly
because I'd have to provide the wire all color-coded and cut to the correct lengths or you wind up
with spagetti. They way MOTM does it is cheaper (gasp!!).

4) Availibility now, and in the future.

This is a REAL issue. The #1 pot supplier is Bourns. They have about 4 different pots that fit the bill. Remember,
in whatever series we pick, we need both linear and log. Ooppsss. 2 of the 4 series don't offer log.

Now we start calling distribution. I am buying 1000 pots (wanting a good price). So, what
do I hear time after time? 14-16 weeks delivery. Hmmm.....

Not wanting to wait 4 months (this is in the third week of Feb. I'm doing all of this) I start looking to other
manufacturers.

The Spectrol pot is available, but 40 cents more expensive (in conductive plastic). BUT, it meets all other
criteria, and there are 1200 sitting in the UK. So, I get a few samples, build some prototypes, and
order. I get the cermet versions for the same price as conductive plastic (beating on the salesman). 
And I like the feel better anyway. And the lower tempco is a bonus. But some values are not in cermet.
So, I buy conductive plastic, for those values.

Also, remember this: the lower cost pots like in Mouser/Digikey may not be around 5 years from now. What
if Mouser switches from Panasonic to Noble to ALPS to Phier? I wanted a pot that I know will be around
15 years from now. The Bourns/Spectrol type pots have been around since the '70s. I still have 20
100K Bourns 91 pots in the original box I found in the attic from 1973. I have a 1981 Spectrol catalog,
and it pretty much looks like the 1998 catalog (Spectrol was a division of Honeywell).

So, I started out looking at the Bourns 91. Ended with the Spectrol 149.

There really is no 'right' answer. The best pot to use is no pot: use an optical shaft encoder with individual
values in little LEDs. Store it all in EEPROM. But that's future MOTM stuff........The Xpander got it right.

The best pot? A wirewound 10-turn. About $40ea. If I had a $15K Moog modular, that's what I'd stick in there
for my VCO pots. Everything else, I'd use a Bourns 91 with metal shafts/bushings.

Lastly, the example I gave of the longevity of the Bourns 91 is valid. These controls are in every Burger King Whopper
and french fry machine. They get hosed down with soap and water twice a day, plus all that grease and high
school kids beating the crap out of them. They also go in saunas and hot tubs. The pot we use when we can't
get delivery on Bourns? Spectrol.

I'd like to hear other recommendations, this is just my little opinion.

BTW: the absolute BEST publication that discusses picking the correct (NOT the most expensive, but correct)
parts in audio circuits is Audio Amatuer magazine, recently changed name to Audio Electronics.
HIGHLY RECOMMENDED. 603-924-9464. Walt Jung is a frequent contributor. Zero BS factor. Everything
they publish is backed up with lab data. Next to Electronotes, the single most valuable source of information.

Paul Schreiber
Synthesis Technology